Resistive coating compositions and resistor elements produced therefrom

ABSTRACT

AN ELECTRICALLY RESISTIVE COATIG COMPOSITION EXHIBITING OUTSTANDING RESISTANCE WHICH COMPRISES AN ADMIXTURE OF A SELECTED HEAT-CURABLE POLYMERIC MATERIAL AND CONDUCTIVE PARTICLES, E.G., CARBON PARTICLES, DISPERSED THEREIN. THESE SELECTED POLYMERIC MATERIALS INCLUDE MIXTURES OF TRIFUNCTIONAL EPOXY RESIN AND PHENOLIC RESIN; MIXTURES OF EPOXY-MODIFIED PHENOLIC RESIN AND PHENOLIC RESIN; MIXTURES OF PHENOLIC RESIN, MELAMINE RESIN, OR PRECURSORS THEREOF, AND EPOXY-MODIFIED PHENOLIC RESIN; MIXTURES OF EPOXY-MODIFIED PHENOLIC RESIN, PHENOLIC RESIN, AND EPOXY RESIN; AND MELAMINE RESINS.

United States Patent 3,686,139 RESISTIVE COATING COMPOSITIONS AND RESIS- TOR ELEMENTS PRODUCED THEREFROM Irving Lubin, Milwaukee, Wis., assignor t0 Globe-Union Inc., Milwaukee, Wis.

No Drawing. Filed Mar. 10, 1970, Ser. No. 18,343 Int. Cl. C09c 1/44; H01b 1/06' U.S. Cl. 252--511 8 Claims ABSTRACT OF THE DISCLOSURE An electrically resistive coating composition exhibiting outstanding abrasion resistance which comprises an admixture of a selected heat-curable polymeric material and conductive particles, e.g., carbon particles, dispersed therein. These selected polymeric materials include mixtures of trifunctional epoxy resin and phenolic resin; mixtures of epoxy-modified phenolic resin and phenolic resin; mixtures of phenolic resin, melamine resin, or precursors thereof, and epoxy-modified phenolic resin; mixtures of epoxy-modified phenolic resin, phenolic resin, and epoxy resin; and melamine resins.

BACKGROUND OF THE INVENTION This invention relates to electrically resistive coating compositions especially suitable for the production of resistors and the like electrical elements. More particularly, this invention relates to electrically resistive plastic coating compositions capable of adhering to dielectric substrates and of providing wear resistance coatings on the substrates and to the resistor elements manufactured from the coated substrates.

Heretofore, it has been known to produce resistive coatings from flowable conductive plastic materials containing electrically conductive particles dispersed within a solvent solution of a heat-curable polymeric material. Also, many attempts have been made to use such conductive plastic materials to produce resistive coatings suitable for the manufacturing resistor elements used in potentiometers and like electrical devices where the resistance of the coating is resolved by a wiper element or similar contact means when moved over the coating.

Many different heat-curable polymeric materials have been used to produce such resistive coatings. Among those most commonly used are phenol-formaldehyde condensates and difunctional epoxy resins. Also mixtures of these epoxy resins with phenol-formaldehyde condensates, melamine resins or urea-formaldehyde condensates and mixtures of phenoland urea-formaldehyde condensates have been proposed.

In production of carbon-containing resistor elements, it is often necessary to produce a plurality of side-byside resistive coatings, each containing particles of different conductivity electrically joined together by electrical junctions between their adjacent edges. For example, one of the resistive coatings may contain metal particles, e.g., silver, to provide a termination zone of low resistivity; the adjacent coating may contain substantially less conductive particles such as carbon to provide a resistor zone of high resistivity and the next adjacent coating may contain metal particles, e.g., silver, to provide another termination zone.

In order to obtain coated substrates having two or more coatings with the required electrical performance characteristics, the adjacent coatings which are of substantially difierent resistivity should have an electrical junction between them that provides a continuous electrical path with an even or smooth transition in resistance from one coating to the other. This requirement for a smooth electrical transition zone between coatings of different resistivity is particularly manifest in the production of resistor elements for potentiometers wherein a wiper element is moved across the resistor element, i.e., from one coating to another. Various methods have been used for applying such resistive coatings including spraying, freeflow film extrusion, brushing, roller coating and the like. One particularly effective method for producing coated substrates having especially smooth electrical junctions between the adjacent coating is disclosed in the commonly assigned application of Ralph E. Mishler filed concurrently herewith and entitled Production of Resistive Coatings.

It has been found that the conductive plastic materials heretofore used for producing resistive coatings for manufacture of resistor elements often do not exhibit the wearresistance required to insure a long service life in a potentiometer. These coatings rarely have sufficient wear resistance to last more than about 5,000 cycles of the wiper element. As used herein, the term cycle refers to the amount of contact required for the wiper element to traverse a resistor element of a given size from one end to the other and then to return to the one end.

SUMMARY OF THE INVENTION Advantageously, this invention provides resistive coating compositions which form resistor elements having substantially greater wear resistances than those heretofore known.

Thus, this invention contemplates an electrically resistive coating composition capable of exhibiting outstanding abrasion resistance, and of adhering to many different dielectric substrates which comprises an admixture of from about 30 to by weight of selected heat-curable polymeric materials and from about 5 to 70% by weight of finely divided conductive particles dispersed therein. Because the polymeric materials used as a binder and dispersant for the conductive particles are heat curable and also because the conductive particles should be uniformly dispersed throughout the admixture, it had been found particularly advantageous that the compositions of this invention should be prepared as solvent solutions. Thus, it will be understood that these coating compositions, which are applied as flowable materials to a dielectric substrate, dried and then cured by heating, will normally contain minor amounts of solvent after the curing step.

In accordance with this invention, it has been found that uniquely wear-resistant coatings for resistor elements can be produced by employing coating compositions containing selected polymeric materials. These selected materials include:

(1) mixtures of trifunctional epoxy resin and phenolic resin;

(2) mixtures of epoxy-modified phenolic resin and phenolic resin;

(3) mixtures of phenolic resin, melamine resin or precursors thereof, and epoxy-modified phenolic resin;

(4) mixtures of epoxy-modified phenolic resin, phenolic resin, and epoxy resin; and

(5) certain melamine resins.

The above-mentioned polymeric materials are especially suitable for preparing coating compositions containing highly electrically resistive particles such as carbon particles. However, in certain cases, such as when the conductive particles in the coating composition are metallic particles, e.g., silver, and the like, it is more difiicult to obtain the desired abrasion-resistant properties of the compositions of this invention. Advantageously, it has been found that the addition of certain solid-type lubricants to the coating composition will increase the wear resistance of these and other coating compositions sufficiently that the desired abrasion resistance can be obtained.

Suitable solid lubricants for use in the coating compositions of this invention include molybdenum disulfide, tungsten disulfide, boron nitride, niobium selenide, tungsten selenide, titanium telluride, and mixtures thereof, and the like, which are unreactive with the polymeric material or the conductive particles at the conditions required for cur ing the coatings onto the substrate. Also, the lubricant must not substantially affect the electrical characteritsics of the coatings. Generally, at least, about by weight of the solid lubricant based on the total solid content is needed to produce a noticeable improvement in the abrasion resistance of the coating composition.

It has been found that the coating composition containing from about to about 90% of a lubricant such as molybdenum disulfide, which is essentially dielectric in character, will produce particularly effective resistive coatings suitable for the purposes of this invention. Preferably, the amount of molybdenum disulfide should be about equal to the amount of metal particles employed in the coating composition.

The resistive coating compositions of this invention contain finely divided electrically conductive particles dispersed uniformly throughout a substantially non-conductive heat-curable polymeric vehicle. The polymeric vehicle must adhere to the dielectric substrate during the application operation and provide a hard solid matrix in which the conductive particles will remain dispersed after curing at elevated temperatures. Examples of suitable polymeric materials include heat curable melamine resins or precursors thereof, such as melamine-formaldehyde condensates, methylated melamine-formaldehyde condensates, butylated melamine-formaldehyde condensates, butylated urea-formaldehyde condensates; phenolic resins, such as phenol-formaldehyde condensates; epoxy resins and epoxy-modified phenolic resins and mixtures thereof. It will be appreciated that several of these heat curable polymeric materials may require hardeners or catalysts to accelerate the curing reaction. As heretofore noted, with the exception of selected melamine resins, such as Resimene 876, it has been found that particular combinations or mixtures of these polymers must be employed to obtain the unique coating compositions of this invention.

Advantageously, these combinations of polymeric materials will cross-link with each other. Epoxy resins will cross-link with phenol-formaldehyde condensates and also with melamine formaldehyde condensates. It will be understood that the phenolic resins employed by this invention are those phenol-formaldehyde condensates which include heat fusible, phenolic novolak resins and the heat-curable, one-step phenolic resins. The novolaks usually are prepared by using a molar ratio of formaldehyde to phenol of less than about 1 to 1 in the presence of a catalyst that is preferably acidic under appropriate reaction conditions. Novolaks are permanently fusible and soluble and do not themselves pass into a cross-linked state.

In order to make the novolak resin infusible and capable of being cured by heat, it must be further reacted with an aldehyde donor or a source of methylene bridges or linkages. The methylene bridges may be provided by compounds which generate formaldehyde which in turn subssequently provides additional methylene bridges between adjacent phenolic nuclei.

The one-step phenolic resins are prepared with a larger mole ratio of formaldehyde to phenol than is employed to prepare the novolaks. Under the influence of alkaline catalysts, phenol reacts with aqueous formaldehyde to attach hydroxymethyl (methylol) groups to form one to all three of the phenolic ortho and para positions with or without the establishment or methylene linkages between phenolic nuclei. Suitable phenolic resins which are commercially available include Bakelite BKS 2710, Varcum 1281 B 65, and BRPA 5570. These resins may be cured to the thermoset (cross-linked) condition by the application of heat alone, but this cure often does not proceed rapidly enough. Consequently, hardner compounds may be used to accelerate the rate of cure.

The hardner compounds capable of being aldehyde donors include hexamethylenetetramine, paraformaldehyde, sym-trioxane and the like. Preferably the hardner is hexamethylenetetramine which is a product of ammonia and formaldehyde. These hardner compounds are considered aldehyde donors in that they effect rapid crosslinking of heat fusible novolak resins and the one step phenolic resins with methylene or equivalent linkages by the application of heat.

Suitable epoxy-modified phenolic resins are exemplified by those made commercially available by Reichhold Chemicals, Inc. and sold as Plyophen 23-983.

The epoxy resins suitable for this invention include polymeric reaction products of polyfunctional halohydrins with polyhydric phenols. Such resins are known in the art as epoxy, epoxides, glycidyl ethers, or ether-epoxides. Among the polyfunctional halohydrins that may be employed to produce the epoxy resins are epichlorohydrin, glycerol dichlorohydrin and the like. Typical polyhydric phenols are the resorcinols and the 2,2-bis (hydroxyphenyl) alkanes, i.e., compounds resulting from the condensation of phenols with aldehydes and ketones including formaldehyde, acetaldehyde, propionaldehyde, acetone, and the like. The epoxy resins often contain terminal epoxy groups but also may contain both terminal epoxy groups and terminal hydroxyl groups. Trifunctional epoxy resins such as Bakelite ERL 0510, sold by Union Carbide, are particularly effective when admixed with phenolic resins.

Many different commercially available epoxy-type resins may be employed to prepare the resistive coating compositions of this invention. These resins include the epoxy resins marketed by the Bakelite Company under the trade names ERL 2774, ERL 4221, and ERL 3794, and BXKS 4466; the Epon resins sold by the Shell Chemical Corporation, i.e., Epon 1001, Epon 1004, Epon 1007, Epon 1009, and Epon 828; those sold by Ciba Company, Inc., designated as Araldite 6010 and 6020; and the GenEpoxy Resins sold by General Mills Chemical Division, i.e., Gen- Epoxy 175, 190, and 525.

In addition to the conventional epoxy resins, other epoxy intermediates and modified epoxy resins may be employed to produce the coating compositions of this invention. Unox Epoxide 201, a product of Union Carbide Chemicals Company, is representative of the new cycloaliphatic epoxy resins that are useful. The modified epoxy resins often contain reactive diluent such as styrene oxide, octylene oxides, allyl glycidyl ether, butyl-glycidyl ether, phenyl glycidyl ether, and the like reactive compounds in amounts varying up to about 20 to 30 parts of diluent per parts of the epoxy resin. Examples of such modified epoxy resins that are commercially available are Bakelite ERL 2795, ERL 4289, ERL 2774, Araldite 502, Gen- Epoxy M-180, and Epon 815. It will be appreciated that the term epoxy resin as herein employed is meant to include the conventional epoxy resins hereinabove described and also those modified epoxy resins and intermediate epoxy resins.

In producing the selected combinations or mixtures of resins useful in the coating compositions of this invention, it has been found that the proportions of each may be varied.

With those coating compositions containing a mixture of a trifunctional epoxy resin such as Bakelite ERL 0510, and phenol-formaldehyde condensate resin such as Bakelite BKS 2710, the weight ratio of the amount of the phenolic resin to the amount of epoxy resin may range from about 3:1 to about 5:1; preferably a ratio of about 4:1 is employed.

The mixtures containing epoxy-modified phenolic resin (such as Plyophen 23-983) and phenolic resin (such as Bakelite BKS 2710) usually have these resins, respectively, in a weight ratio of from about 1:2 to 1:1 and preferably in a ratio of about 1:15 If an epoxy resin is added to this mixture it is generally added in the same Weight proportion as the epoxy-modified phenolic resin. For example, a particularly effective combination of phenolic resin, epoxy-modified phenolic resin and epoxy resin is one in which these resins are respectively in a weight ratio of 1.5 1 1. Lesser amounts of epoxy resins may also be used.

When melamine resins or melamine resin precursors such as hexamethoxymethylmelamine are added to these phenolic resin mixtures, the melamine resin is usually employed as a curing or cross-linking aid. Consequently, it is utilized in relatively small amounts. Thus, these mixtures may contain about 50 to 65% phenolic resin, 30 to 50% of the epoxy-modified phenolic resin, and about 5 to of the melamine resin, based on the total weight of the mixture.

If the melamine resins are used alone, they may range from about 30 to about 60% by weight of the total weight of the solids content of the coating composition.

If the melamine resins are used alone, they may range from about 30 to about 60% by weight of the total weight of the solids content of the coating composition.

It will be understood that the amount, type and size of the conductive particles used in the flowable conductive plastic materials determines the resistivity of the material. Because of their varied conductivity, carbon particles have been found to be particularly effective for producing resistive coatings. The carbon particles will comprise from about 4 to about 60% by weight of the flowable solventcontaining coating composition. Preferably, from about 7 to about 30% by weight of the carbon particles are employed to produce flowable compositions of suitable viscosity. When the carbon particle content is above 60% by weight, the viscosity of the flowable coating composition is often too high for effective application, e.g., by doctoring. Below a carbon particle content of about 7% by weight, the polymeric voids formed between the carbon particles after curing or cross-linking of the polymeric vehicle have an adverse effect on the electrical characteristics of the coating. For example, it has been found that the noise level of the resistive coating will be excessively high and therefore not commercially acceptable at this low carbon content.

It has been found that metal particles such as those of silver, platinum, other noble metals, copper, stainless steel and the like, may also be used as conductive particles in the coating material of this invention. Such metal-containing materials are especially useful for forming the terminations of a resistor element. Depending on the metals employed and the resistivity desired, the metal particle content may vary considerably. It has been found that silver particles in amounts from about 30% up to about 50% by weight of the solvent-containing composition are especially suitable for producing a termination zone having a resistance that is 1% of the total resistance of the resistor element. It will be appreciated that lower amounts of the metal particles, e.g., 10% by weight, or higher amounts of the metal particles, e.g., 65 may be also used for producing different resistive coatings.

Metal particles have a less pronounced affect on the viscosity of the resistive plastic materials. The primary consideration which determines the maximum amount of metal particles used is capability of the polymeric vehicle in the plastic material to adhesively bond the particles to the substrate to be coated.

In general, from 0.25 to 1 part by weight of the resinous vehicle is required to 1 part of metal by weight, and from about 0.5 to 2 parts of the resin per 1 part of carbon by weight.

Resistive plastic coating materials containing either all carbon or all metal particles are normally used, but mixtures of each or both may be employed.

The carbon particles used may be in the various forms, i.e., crystalline or amorphorous, found in commercially available carbons such as acetylene blacks or furnace blacks. Often the carbon particles are calcined in air at elevated temperatures on the order of 2,000 to 3,000 F. for several hours prior to use in the preparation of the conductive plastic materials. The carbon particles may range in size from about 10 to about 400 millimicrons and mixtures of larger and smaller particles may be used.

Metal particles, on the other hand, are usually considerably larger than carbon particles and may have particle sizes ranging from about 10 'to about 400 microns.

It will be appreciated that the resistivity of the conductive, particle-containing plastic material is determined by the amount of conductive particles used; the resistivity varying inversely to the amount of particles.

Because many of the thermosetting polymers or mixtures thereof used as the vehicle or binder for the conductive particles may have viscosities which are higher than desired for the purposes of the invention, it is often necessary to use an organic compound, which is a solvent for the polymer, to regulate the viscosity of the conductive plastic material. These solvents should be nonreactive with the polymeric vehicle and must be sufficiently volatile to be removed from the applied coating by evaporation. Exemplary of suitable solvent materials are aliphatic alcohols such as ethanol, and isobutyl alcohol, and the like; aliphatic ketones such as methyl ethyl ketone, methyl isobutyl ketone, and the like; cyclic ketones such as isophorone; glycol ethers such as ethylene glycol n-butyl ether, ethylene glycol ethyl ether, and the like; as well as aromatic hydrocarbons such as benzene, toluene, the xylenes, and the like. Since, as heretofore noted, the solvents are primarily used to regulate the viscosity of the resistive plastic materials, the amount of solvent may vary considerably, i.e., from about 5 to about 70% by weight of the flowable resistive coating material to be applied to the substrate.

It will be understood that various additives and other compounding aids may be used in preparation of the resistive coating materials in order to facilitate their application on to a dielectric substrate. For example, it has been found that silicone oils and other similar surfactants may be employed to prevent the occurrence of surface imperfections in the coating. Also, polymeric thickening agents, metal cross-linking inhibitors such as catechol, and the like, may be used. Usually, such additives will comprise a relatively small amount, i.e., about 1 to 5 parts by weight of the coating composition. Also, catalysts and hardeners for the polymeric vehicle as heretofore described will be added during compounding of the coating materials.

The viscosity of the resistive coating materials will be sufficiently high to insure formation of uniform edges on each coating and must be in the range that allows the material to flow on to the dielectric substrate. Resistive plastic materials having viscosities of from about centipoises to as high as 80,000 centipoises can be used (these viscosities are obtained at a spindle speed of 10 r.p.m. on a Brookfield Viscosimeter). It will be understood that the manner in which the coatings are applied, i.e., by spraying, roller coating, doctoring, brushing, silk-screening, etc., determines the minimum viscosity of the plastic material that can be used. In general, viscosities of at least about 200 cps. (at a Brookfield spindle speed of 10 r.p.m.) are needed for doctoring applications. Preferably, these viscosities range from about 3,000 to about 20,000 cps.

In addition, when using a doctoring technique, it is preferable to utilize resistive coating materials which exhibit thixotropic properties. These materials are more readily applied by a doctoring edge. Also they insure that the edges between adjacent coatings will provide the even electrical transition from one resistive coating to another.

In general, the index of thixotropy of these materials may vary from about 1.5 to about 40 and preferably from about 2 to about 20. It will be understood that as used herein the term index of thixotropy refers to the ratio of the viscosity of a material at one level of agitation to the viscosity at another level. Many of the polymeric materials suitable for this invention may show -viscosities of from about 200,000 up to about 800,000 or more when measured at a spindle speed of 0.5 r.p.m. on a Brookfield Viscosimeter.

The substrate or base to be coated in accordance with this invention is a dielectric, insulating material which must be stable under the conditions required for fixing the coatings to the surface of the substrate. Examples of some of these materials are sheets, strips, films, and the like, formed from polymers, e.g., phonelic resins, polyvinyl chloride, polyethylene, epoxy resin, and the like; glass, ceramics, treated papers, and the like. It will be appreciated that the substrate may be stationary Or advanced under a doctoring edge or roller or other applicator as successive sheets or strips in end-to-end abutment or as a continuously flexible film which may be withdrawn on a take-up roll.

The coating compositions of this invention may be applied to the dielectric substrate to produce micro-thin coatings having thicknesses of from about 0.5 to about 20 mils. Because of the solvent usually present in the fiowable coating composition, the coatings will be thinner after removal of the solvent, e.g., by drying. The wet coatings may shrink as much as about 60% or more after being dried and fixed to the substrate. Consequently, the solvent-free coatings may have thicknesses varying from about 0.25 to about mils.

It will be understood that the coating compositions of this invention must be heat-cured to fix the coating to the dielectric substrate. Usually, the coated substrate is initially heated to dry, or precure, the coating at temperaures of from 200 to 300 F. for a few minutes and then heated to temperatures of from 250 to 350 F. or higher for periods of /2 hour to 4 hours to cure the polymeric vehicle.

It has been found that the wear-resistant coatings of this invention have different micro-hardness when tested, for example, by a Knoop indenter. Thus, the composition-containing phenolic resins usually produce cured coatings having hardness readings in the range of about 60 to about 70; whereas the melamine-type compositions produce coatings having hardness readings of from about to about 30. (These readings were obtained with a gram load and with polished samples of the coatings.)

After the coatings have been fixed to the substrate, resistor elements for the manufacture of potentiometers and other variable resistors may be produced by stamping, cutting or otherwise blanking out a plurality of resistive elements from the coated substrate. These elements may be in various shapes, e.g., sector-shaped, circular segments, crescent-shaped, rectangular, and the like. The manner in which these resistive elements are formed will be hereinafter described in greater detail. Also, the specific shape of certain sector-shaped elements is illustrated in the heretofore noted application of Ralph E. Mishler.

DESCRIPTION OF THE PREFERRED EMBODIMENT The coating composition of this invention will be more readily understood by reference to the following examples:

EXAMPLE I This example illustrates preparation of a resistive coating composition containing a mixture of phenolic resin, epoxy and epoxy-modified phenolic resin for manufacturing carbon-containing resistive elements of a potentiometer.

This coating composition contained the following ingredients in weight percentage:

Ingredients: Percent by weight Carbon particles 1 23.2 Phenolic resin 2 20.6 Epoxy-modified phenolic resin 3 12.7 Epoxy 4 12.5 Hexamethylenetetramine 1.4 Methyl ethyl ketone 6.0 Isophorone 23 .5 9 Fluorocarbon surfactant 5 .01

1 Carbon particles having a diameter of 54 ma, Stat'cx 02+- a product of Columbian Carbon C0.

-56% solution of resin in ethanol, BKS 2710a product of Union Carbide Co.

3 60% Solution of resin in isopropyl alcohol, Plyophen 233- 983a product of Reichhold Chemical Co.

*55% solution of resin in mixture of methyl isobutyl ketone and toluene, Epon 1007a product of Shell Chemical Co. IgXa product of Minnesota Mining & Manufacturmg 0.

These ingredients were mixed in a Dispersator blending device and thereafter ball milled for about 24 hours until the carbon particles were uniformly dispersed in the resins and solvents to form a flowable plastic coating material. Then a dielectric substrate, i.e., a phenolic strip, produced by the Synthane Corporation, having a dielectric strength greater than 1,000 megohms, a thickness of 0.020 inch, and a length of approximately 28 inches was coated by doctoring the coating composition to a thickness of 3 mils wet.

The coated phenolic strip was then dried for 10 minutes at 200 F. and cured for 1 hour at 325 F. to secure the coating to the strip.

A plurality of identical sector-shaped resistor elements, each having a resistance of about 500 ohms, was then cut out of the coated substrate by a blanking die. These elements each had an outer diameter of approximately 0.550 of an inch and a radial width of about 0.117 of an inch and included angle of about 62 between the centers of the terminal openings. This element is of the size and type used in a Centralab Model 3 potentiometer (Centralab is a trademark of Globe-Union, Inc.). In order to evaluate the wear or abrasion resistance of the resistive coating on the resistor element, a contact resistance variation test method designated by Centralab Specification No. 04BB1 was conducted.

In this test the resistor element is placed in a Model 3 potentiometer assembly having a silver-clad Phosphor bronze wiper element which contacts the resistor element at a pressure of approximately 100,000 p.s.i. The assembly is then mounted on a testing apparatus which rotates the wiper element back and forth across the resistor element at a rate of 50 cycles per minute, the total number of cycles being recorded by a counter. The potentiometer assembly is also connected electrically to an electronic resistance measuring apparatus including an X-Y plotter which plots the noise level i.e. the variation in resistance across the element at a constant current of 1 milliamp. This test is usually continued until the noise level exceeds a level of 1% of the total resistance or until the surface of the coating shows excessive groove formation or damage to the wiper is noted by periodic visual examination. Upon evaluating resistor elements formed in this example by this test procedure, it was found that the coating on the resistor element showed a cycle life greater than 90,000 cycles without any appreciable surface deformation on itself or on the wiper element. The test was then discontinued.

EXAMPLE II This example illustrates the wear resistance and improvedcycle life obtained by the coating compositions containing selected melamine resins as a polymeric vehicle for carbon particles. The following coating composition was used to prepare a ilowable resistive coating:

1 Carbon particles having diameters FS-a product of Cabot Carbon C0.

2 Air calcined.

3 Carbon particles having diameters of 17 mo, Conductex SC-a product of Columbian Carbon C0.

4 Carbon particles having diameters of 42 Ill/L, Sterling V a product of Cabot Carbon Co.

56% solution of resin in isobutyl alcohol, Resimene 876- a product of Monsanto Co.

of 60 mp, Regal SR In preparing this composition, the para-toluene sulfonic acid, methyl ethyl ketone, and the ethylene glycol and the n butyl ether were added to a one-gallon can and then mixed on a Dispersator until the sulfonic acid was dissolved. Then the melamine resin was added and mixed in. Subsequently, the carbon particles were added and mixed for one-half hour. The resulting admixture was then ball milled for 16 hours.

The coating composition obtained was applied to a phenolic strip (as used in Example I) to a wet thickness of 3 mils and the strip was dried for 4 minutes at 300 F. and cured for 2 hours at 250 F.

As described in Example I, sector-shaped resistive elements were then cut out of a coated phenolic substrate and tested for their cycle life in the Model 3 potentiometer assembly. From these tests it was found that the instant coating composition produced a resistive coating having a cycle life greater than 5 0,000 cycles.

EXAMPLE HI The use of a resistive coating containing a mixture of phenolic resin and trifunctional epoxy resin as a polymetric vehicle for carbon particles is illustrated by this example.

A resistive coating material having the following composition was prepared:

This coating composition was admixed by first dissolving the phenolic resin and the epoxy resin in the solvent mixture and by then ball milling for 16 hours.

The resulting flowable coating material was applied at a thickness of 3 mils wet to a phenolic strip (of the type in Example I) by spraying through a nozzle at 30 psi. atomizing pressure. The strip was then dried at 200 F. for minutes and cured for V2 hour at 325 F.

Sector-shaped resistor elements suitable for use in a 10 Model 3 potentiometer were cut out and tested as in Example I. From these tests it was found that this composition produced a resistive coating having a cycle life greater than 50,000 cycles.

EXAMPLE IV This example illustrates the use of a coating composition capable of forming a resistive coating having exceptional extended cycle life in which the polymeric vehicle is a mixture of phenolic resin and epoxy-modified phenolic resin.

This coating composition contained the following ingredients by weight percent:

Ingredients: Percent by weight Carbon particles 1 2 21.6

Carbon particles 3 2 5.4 Ph'enolic resin 29.5 Epoxy-modified phenolic resin 5 17.2 Methyl ethyl ketone 6.0 Isophorone 20.3

1 Statex 93.

2 Air calcined.

3 Conductex SC.

4 BKS 2710.

5 Plyophen 23-983.

In preparing this coating composition, a 1000 gram batch was made by mixing the phenolic resin and the epoxy-modified phenolic resin, methyl ethyl ketone, and isophorone in a one-gallon can for 5 minutes. Then the carbon particles were added and blended for /2 hour and the resulting admixture was ball milled for 19 /2 hours. This coating was then doctored onto a phenolic strip at a thickness of 3 mils wet and then dried for 4 minutes at 300 F. and cured for 2 hours at 325 F.

As in Example I, sector-shaped resistive elements suitable for a Model 3 potentiometer assembly were blanked out from the coated phenolic strip and then mounted in assembly. Tests of these sectors showed that the coating composition gave a cycle life in excess of 400,000 cycles without causing any appreciable wear to the surface of the coating or damage to the wiper element.

EXAMPLE V This example illustrates the use of solid lubricants, such as molybdenum disulfide, for preparing the unique resistive coating compositions of this invention.

A silver-containing coating composition, suitable for producing a metal termination zone on a resistor element was prepared using the following ingredients:

1 #750-11 product of Metal Disintegrating Co. 2 BKS 2710.

3 Plyophen 23983. 4 Cymel 301-a product of American Cyanamid.

'In accordance with the procedure outlined in Example I this termination material was applied to a phenolic strip and sector-shaped resistor elements were blanked out and then tested for their rotational life. It was found these coatings also exhibit a cycle life greater than 50,000 cycles.

1 1 EXAMPLE v1 This example further illustrates the unique advantages of employing solid lubricants to enhance the abrasive resistance of metal-containing resistive coatings. Initially, a coating material containing the following ingredients was prepared:

Ingredients: Percent by weight Silver flake 40.0

Phenolic resin 26.8 Epoxy-modified phenolic resin 3 15.7 Hexamethoxy-methylmelamine 4 1.8 Catechol 0.5

Polymeric thickener 5 0.25 Isophorone 14.95

1 #750-a product of Metal Disintegrating Co.

56% solution of resin in ethanol, BKS 2710a product of Union Carbide.

60% solution of resin in isopropyl alcohol, Plyophen 23- 083a product of Reichhold Chemical Co.

4 A mclamine-formalidehyde condensate, Cyinel 301a product of American Cyanamld.

'lhixotrol STa product of Baker Chemical.

This coating material was then applied by doctoring to a 3 mil wet thickness to the phenolic substrate, dried at 4 /2 minutes at 300 F. and cured for 2 hours at 325 F. Sector-shaped resistor elements suitable for testing in Model 3 potentiometer were then prepared, and then evaluated for their rotational life as described in Example I. It was found from these tests that this coating gave a cycle life less than 5,000 cycles.

Another coating material was prepared which had the following composition:

Ingredients: Percent by weight Molybdenum disulfide 40.0 Phenolic resin 26.8 Epoxy-modified phenolic resin 15.7 'Hexamethoxy-methylmelamine 1.8 Catechol 0.50 Polymeric thickener 0.25 Isophorone 14.95

Same as in preceding table.

The second coating material (which is a composition identical to the first except molybdenum disulfide is substituted for the silver flake) was then mixed in a 1:1 weight ratio with the previously prepared silver-containing composition and applied by doctoring to a thickness of 3 mils wet to the phenolic strips. The strips were then cured at the conditions used for the first silver-containing composition. Evaluation of wear resistance on the coating by the above-noted rotation test showed that the coating gave a cycle life in excess of 50,000.

It will be appreciated that this data illustrate this convenience and adaptability obtaind by the blending of solid lubricant-containing compositions with the silver-containing compositions to obtain the desired wear resistance.

It will be further appreciated that the coating compositions of this invention provide resistive micro-thin coatings having a cycle life of 50,000 or more cycles.

EXAMPLE VII By following the procedures and conditions described in Examle I, many other resistive coatings were produced in which polymeric materials including phenolic resins, mixtures of phenolic and difunctional epoxy resins, polybutadiene, diallyl isophthalate and others were substituted 12 for the selected polymeric materials of this invention. In each case, the resulting resistive coating on the resistor element was found to have a substantially shorter cycle life, i.e. from about 2,000 cycles to 10,000 cycles.

What is claimed is:

1. A heat-curable fiowable electrically resistive coating composition capable of adhering to a dielectric substrate and of producing a microthin coating exhibiting outstanding abrasion resistance after curing when contacted with an electrical wiper, which comprises an admixture of a polymeric vehicle dispersed in a solvent for the vehicle, said solvent comprising from about 5 to about percent by weight of the total weight of the composition and said vehicle being selected from the group consisting of mixtures of trifunctional epoxy resin and phenolic resin; mixtures of epoxy-modified phenolic resin and phenolic resin; mixtures of phenolic resin, melamine resin, or precursors thereof, and epoxy-modified phenolic resin; mixtures of epoxy-modified phenolic resin, phenolic resin, and epoxy resin; and melamine resins; said admixture containing about at least 30 percent by weight of a solid lubricant that does not substantially affect the electrical characteristics of the coating and is based on the total solid content of the admixture; and having finely divided conductive particles homogeneously dispersed therethrough, said vehicle being present in a greater amount by weight than the conductive particles; and said conductive particles having an average particle size of from about 10 millimicrons to about 400 microns.

2. The fiowable coating composition of claim 1 in which the metal particles are silver particles and the solid lubricant is molybdenum disulfide.

3. The coating composition of claim 1 in which the thickness of the microthin coating is from about 0.5 to about 20 mils.

4. The coating composition of claim 1 in which the conductive particles are carbon particles having an average particle size of from about 10 to about 400 millimicrons.

5. An electrical resistor element having a greatly extended service life by resistance to abrasion from wiper contact during use, which comprises a smooth dielectric base covered with a heat cured coating composition formed from an admixture of a polymeric material selected from the group consisting of mixtures of trifunctional epoxy resin and phenolic resin; mixtures of epoxymodified phenolic resin and phenolic resin; mixtures of phenolic resin, melamine resin, or precursors thereof, and epoxy-modified phenolic resin; mixtures of epoxy-modified phenolic resin, phenolic resin, and epoxy resin; and melamine resin; said admixture containing about at least 30 percent by weight of a solid lubricant that does not substantially affect the electrical characteristics of the coating and is based on the total solid content of the admixture; conductive particles dispersed homogeneously therethrough; and a solvent comprising from about 5 to about 70 percent by weight of the total weight of the composition before curing; said polymeric material being present in a greater amount by weight than said conductive particles and said conductive particles having an average particle size of from about 10 millimicrons to about 400 microns.

6. The resistor element of claim 5 in which the metal particles are silver particles and the solid lubricant is molybdenum disulfide.

7. The electrical resistor element of claim 5 in which the thickness of the cured microthin coating is from about 0.25 to about 10 mils.

8. The electrical resistor element of claim 5 in which said conductive particles are carbon particles having an average particle size of from about 10 to about 400 millimicrons.

(References on following page) 13 14 References Cited 2,866,768 12/1958 Bolstad 260-37 EP UNITED STATES PATENTS 3,412,043 11/1968 Gilliland 260-37 EP 10/1962 Pass 252-512 OTHER REFERENCES 6/1967 McKeand et a1 117-226 5/1969 Hubbuch 252 511 5 De lllgnt' Metal Fllled Plast cs Remhold N Y 1961 3/1971 run 252-511 1 2/ 1947 Henry et a1. 2525 11 10/1965 Biggs DOUGLAS J. DRUMMOND, Prlmary Exammer 12/1960 Coad 252---5l1 8/1965 IMcLean et a1. 26037 EP 10 us 3/1958 Silversher 252--511 252512; 260--37 E P; 117226 

